The invention is in the field of carburetors for two stroke or two cycle engines, with particular application to outboard motors. Reducing pressure in the float bowl of the carburetor of a four cycle engine has been disclosed in U.S. Pat. Nos. 1,799,585, 1,805,763, 1,785,681, 2,029,142, 2,752,136, 1,851,711, and 1,740,917, with the first two patents listed being considered most pertinent. However, the concept has not, to the inventor's knowledge, ever before been applied to two cycle engines and particularly not to outboard motors.
Applying the broad concept to two stroke or two cycle engines as herein taught, presented special problems. First, while air pressure is lowest (greatest vacuum) in the intake manifold of a four cycle engine at idle and low RPM and increases with increased RPM, pressure in the crankcase of a two cycle engine is highest (close to atmospheric) at idle, and generally decreases to a low point somewhere in midrange RPM and then increases as RPM further increases, see FIG. 2. Furthermore, the vacuum in the carburetor throat of a two cycle engine is normally much less than half the vacuum in the intake manifold of the four cycle engine at relatively low speeds. See FIG. 2.
The prior art employed the aforementioned low pressure (vacuum) existing in the manifold of the four cycle engine to reduce the pressure in the float bowl. However, to apply the broad theory manifested in the prior art patents to two cycle outboard motors, the inventor had to solve two basic problems. First, compensate for the lower vacuum of the two cycle engine by somehow creating a regulating vacuum at low RPM, and second, program the application of vacuum to the float bowl in a manner so as to take advantage of its economizing effect without adversely effecting the performance of the motor when used to drive a boat. A mating of the knowledge of 2 cycle outboard motor performance and boat hull characteristics was required to achieve the invention.
While the drag of an automobile and a displacement type boat hull increases with speed, the drag on a planing type boat hull increases rapidly when starting up and then decreases for a period when the hull starts to plane upon the water before increasing again as speed is increased. See FIG. 1. It is common practice to use high or full power to get the hull "on plane" and then reduce throttle and travel in the low drag region for the hull, or best cruise range. Experiments have taught that most planing hulls of 16' to 20', average for a recreational boat, achieve the planing condition at speeds of between 15 to 20 miles per hour. Experiments have also taught that when outboard motors are provided with propellers that permit them to run at desired maximum RPM when propelling particular boats, that the RPM required to maintain these boats on plane in the cruising speed range is about the same, and that the corresponding throttle settings (positions of throttle valves in the carburetor throats) will not vary greatly for a particular engine used on different sized boats, if it has been "propped" to achieve full RPM on the particular boat.
Earlier attempts to achieve greater economy in outboard engines led to the development of what has been called the economizer linkage. Broadly, this linkage provides a variable mechanical link between the throttle and spark to provide optimum spark advance for each throttle opening over the full range of engine RPM. A typical profile of throttle and spark settings required for the full RPM range of a representative 150 HP outboard motor is illustrated in FIG. 3. Note that the early increase in RPM results from spark balance alone, the midrange increase from advance of throttle and spark, and advance to high power from opening throttle alone. This limited throttle movement has been found to play a part in the invention.
Until the invention, success in efforts to make outboard motors run on less fuel effectively terminated with the economizer linkage. If it was known that such engines were running rich at cruising speeds, little had been done about it, and it remained for the inventor, based upon his 18 years experience as a carburetor engineer, to conceive of a way to employ the back draft concept illustrated in the 44 year old art cited above on an outboard motor equipped with the economizer linkage and achieve the material improvement in economy illustrated herein.
The invention sprang from the discovery, through testing under operating conditions, that an outboard motor carburetor configured to give good wide open throttle, idle and acceleration performance and operated by an economizer linkage with the spark, provided an unnecessarily rich mixture over the intermediate throttle range e.g. 12.degree. to 30.degree. throttle valve opening. Although the economizer linkage achieved the best throttle/spark relationship for maximum RPM at all throttle settings, the invention achieves a further leaning for greater economy.
The primary objective of the invention is to reduce fuel consumption of 2 cycle engines and outboard motors in particular, and to do so without sacrifice of full power, acceleration or idle performance of the engine, and to accomplish the above in the simplest and most economical manner.
Further objectives of the invention were to provide means for adapting the system of the invention for operation at various altitudes, and to provide a carburetor incorporating the system with means to permit its adaption for use on engines of different horsepower and design.